The cell surface of mammalian cells is in a constant state of flux due to the continuous ingestion of segments of the plasma membrane through the process of pinocytosis. Recent evidence indicates these segments must be conserved and recycled. Two basic approaches will be taken to probe the processing of pinocytic vesicles in fibroblasts. CHO cells will be pulse loaded with pinocytic content markers and the intracellular transport and possible regurgitation of marker quantitated. Preliminary experiments indicate horseradish peroxidase is an effective fluid phase pinocytic marker in pulse periods as short as two and one half minutes. Pinocytic membrane will be selectively labeled by lactoperoxidase mediated radioiodination and the variety and metabolic stability of the radiolabeled proteins determined. Preliminary experiments have established specific conditions for radioiodination and have resulted in data indicating that the electrophoretic spectrum of proteins radioiodinated is different than those accessible to cell surface radioiodination. Moreover, these experiments indicate the proteins turnover in a biphasic manner with an overall half-life of less than 30 hours. Other experiments will be directed towards the route of intracellular transport of radioiodinated pinosome membrane proteins and their possible appearance at the cell surface. Selectively radioiodination of pinosomes will permit a molecular description of the processing of pinosome membrane. Knowledge of how the cell surface turns over should be important in being able to modulate cell interactions and growth. The ability to label membrane proteins included in pinocytic membrane and to follow these proteins will provide a foundation for future projects in cancer research and aging research.